1. Field of the Invention
The present invention relates to an ultrasonic motor, a circuit for driving the ultrasonic motor, and a driving method using the circuit.
2. Description of the Related Art
In the related art, it is known that frequency characteristics of ultrasonic motors vary with changes in temperature, and therefore it is difficult to accurately control the operation of such motors. In general, as the temperature increases, the resonance frequency of an oscillator of an ultrasonic motor decreases and also a maximum rotation speed, which is a rotation speed obtained when the ultrasonic motor is driven at the resonance frequency, increases. When the ultrasonic motor is driven at a frequency lower than the resonance frequency of the oscillator of the ultrasonic motor, the operation becomes unstable, the rotation speed significantly decreases, and controllability deteriorates. In addition to this, in some cases, the flow of a large current damages the ultrasonic motor or a driving circuit.
Usually, when the ultrasonic motor in a resting state starts to be driven, an alternating-current driving voltage having a driving frequency, which is higher than the frequency to start the ultrasonic motor, is firstly applied. Then, the driving frequency is decreased to start rotating the ultrasonic motor and thus to adjust the rotation speed. In general, an operating temperature range of the ultrasonic motor is determined and a stable operation in the operating temperature range is demanded.
Thus, variations, due to temperature, in the resonance frequency of the oscillator of the ultrasonic motor have to be dealt with by using some technique and the motor has to be driven at a frequency higher than the resonance frequency at all times.
As a method to solve the above drawback, Japanese Patent Laid-Open No. 7-59367 discloses a method in which a memory that stores data regarding a lower limit frequency fL, which is a frequency higher than a resonance frequency at a lowest temperature of a predetermined operating temperature range, and an oscillation control circuit that performs control so that a frequency is higher than the lower limit frequency are used. This lowest temperature means a lower limit temperature in an operating temperature range that is estimated by the user.
FIG. 10 is a conceptual diagram illustrating a driving method for an ultrasonic motor described in Japanese Patent Laid-Open No. 7-59367. The estimated operating temperature range is −20° C. to +60° C. and FIG. 10 illustrates a relationship between a driving frequency f and a rotation speed N at typical temperatures of −20° C., +20° C., and +60° C. A common example of the relationship between the rotation speed of the ultrasonic motor and the applied driving frequency is illustrated in FIG. 10. As shown in the drawing, when the temperature increases, the resonance frequency decreases and the rotation speed at the resonance frequency increases.
In the case of driving at the lower limit frequency fL, suppose now that the rotation speeds at −20° C., +20° C., and +60° C. are NmL, NmM, and NmH, respectively, and the resonance frequencies at −20° C., +20° C., and +60° C. are frL, frM, and frH, respectively. Further, in the case of driving at the resonance frequencies at the respective temperatures, suppose now that the rotation speeds are NrL, NrM, and NrH. The terms, a highest temperature, means an upper limit temperature in the operating temperature range that is estimated by the user.
The lower limit frequency fL is set to a frequency at which a rotation speed close to the maximum rotation speed NrL is obtained with stability at the above-set lowest temperature, and the frequency data is stored in the memory. In this state, the oscillation control circuit performs driving at a frequency higher than the lower limit frequency fL on the basis of information stored in the memory. When driving is performed by using such a method, the driving frequency does not fall in a region of frequencies lower than all the resonance frequencies frL, frM, and frH over the entire temperature range.
As another method to solve the above drawback, Japanese Patent Laid-Open No. 6-296378 discloses a method in which a value of such a lower limit frequency is corrected on the basis of a temperature detected by a temperature sensor arranged in proximity to an ultrasonic motor.
FIG. 11 is a conceptual diagram illustrating changes in the lower limit frequency to be set on the basis of a temperature, which is described in Japanese Patent Laid-Open No. 6-296378.
As described above, because the resonance frequency of the ultrasonic motor varies with changes in temperature, the lower limit frequency is set with consideration of the temperature dependence of the resonance frequency. Specifically, data of the lower limit frequencies at temperatures illustrated in FIG. 11 are stored in a memory. Because the resonance frequency shifts to the lower frequency side with increasing temperature, the value of the lower limit frequency is set so as to decrease in accordance with that shift. When control is performed, the lower limit frequency corresponding to a temperature detected by the temperature sensor is read from the memory and is set. Because the lower limit frequency is set so as to be higher than the resonance frequency at each temperature, the driving frequency does not fall in a region of frequencies lower than the resonance frequencies in the respective temperature conditions.